diff --git a/src/Bundle.js b/src/Bundle.js
index accd9ae..c6a2386 100644
--- a/src/Bundle.js
+++ b/src/Bundle.js
@@ -63,7 +63,7 @@ export default class Bundle {
 		this.onwarn = options.onwarn || makeOnwarn();
 
 		// TODO strictly speaking, this only applies with non-ES6, non-default-only bundles
-		[ 'module', 'exports' ].forEach( global => this.assumedGlobals[ global ] = true );
+		[ 'module', 'exports', '_interopDefault' ].forEach( global => this.assumedGlobals[ global ] = true );
 	}
 
 	build () {
diff --git a/src/finalisers/cjs.js b/src/finalisers/cjs.js
index 4651409..c71f59d 100644
--- a/src/finalisers/cjs.js
+++ b/src/finalisers/cjs.js
@@ -6,18 +6,19 @@ export default function cjs ( bundle, magicString, { exportMode }, options ) {
 	const hasDefaultImport = bundle.externalModules.some( mod => mod.declarations.default);
 
 	if (hasDefaultImport) {
-		intro += `function _interopRequire (id) { var ex = require(id); return 'default' in ex ? ex['default'] : ex; }\n\n`;
+		intro += `function _interopDefault (ex) { return 'default' in ex ? ex['default'] : ex; }\n\n`;
 	}
 
 	// TODO handle empty imports, once they're supported
 	const importBlock = bundle.externalModules
 		.map( module => {
 			if ( module.declarations.default ) {
-				let importStatement = `var ${module.name} = _interopRequire('${module.id}');`;
 				if (module.exportsNames) {
-					importStatement += `\nvar ${module.name}__default = ${module.name};`;
+					return `var ${module.name} = require('${module.id}');` +
+						`\nvar ${module.name}__default = _interopDefault(${module.name});`;
+				} else {
+					return `var ${module.name} = _interopDefault(require('${module.id}'));`;
 				}
-				return importStatement;
 			} else {
 				return `var ${module.name} = require('${module.id}');`;
 			}
diff --git a/test/form/external-imports-custom-names/_expected/cjs.js b/test/form/external-imports-custom-names/_expected/cjs.js
index 7c31c60..8f6506e 100644
--- a/test/form/external-imports-custom-names/_expected/cjs.js
+++ b/test/form/external-imports-custom-names/_expected/cjs.js
@@ -1,9 +1,9 @@
 'use strict';
 
-function _interopRequire (id) { var ex = require(id); return 'default' in ex ? ex['default'] : ex; }
+function _interopDefault (ex) { return 'default' in ex ? ex['default'] : ex; }
 
-var $ = _interopRequire('jquery');
+var $ = _interopDefault(require('jquery'));
 
 $( function () {
 	$( 'body' ).html( '<h1>hello world!</h1>' );
-});
+});
\ No newline at end of file
diff --git a/test/form/external-imports/_expected/cjs.js b/test/form/external-imports/_expected/cjs.js
index 6cfae7f..a3ee9b0 100644
--- a/test/form/external-imports/_expected/cjs.js
+++ b/test/form/external-imports/_expected/cjs.js
@@ -1,12 +1,12 @@
 'use strict';
 
-function _interopRequire (id) { var ex = require(id); return 'default' in ex ? ex['default'] : ex; }
+function _interopDefault (ex) { return 'default' in ex ? ex['default'] : ex; }
 
-var factory = _interopRequire('factory');
+var factory = _interopDefault(require('factory'));
 var baz = require('baz');
 var containers = require('shipping-port');
-var alphabet = _interopRequire('alphabet');
-var alphabet__default = alphabet;
+var alphabet = require('alphabet');
+var alphabet__default = _interopDefault(alphabet);
 
 factory( null );
 baz.foo( baz.bar, containers.port );